A feed fluid such as waste water, can be separated into permeate, such as pure water which passes through a membrane, and concentrate which includes water with a high concentration of particles. Such separation can be accomplished by the use of a stack of membrane packs lying within a container. Fouling of the membrane packs, by the buildup of particles at the surface which block pores of the membranes, can be reduced by rapidly rotating the membrane packs, as described in U.S. Pat. No. 4,025,425 by Croopnick. Fouling can be further reduced by placing stationary separator elements between pairs of membrane packs, to create turbulence in the gap between the rotating surface of the membrane pack and the stationary surface of the separating element. It is noted that when a membrane pack has large pores (many microns wide), it may be referred to as a filter pack, but applicant uses the term membrane pack herein for both.
The turbulence-enhancing separator elements lying between membrane packs should be relatively thin to take up little space for a high density of filtration or membrane pack surface. However, the separator elements must not touch the rapidly rotating membrane packs or they will destroy them. It would be desirable if the separator elements could be designed for maximum strength against deflection while minimizing conditions that could cause their deflection.
A common filtration construction directs the feed fluid in series through the gaps. For example, if there is a stack of fifty membrane packs and corresponding stationary elements to produce one hundred gaps, the fluid may flow in a series serpentine path through the one hundred gaps. Such serial flow has the advantage that the feed fluid moves along a long path in contact with the surfaces of the membrane packs, to remove a considerable portion of the filtrate. However, such serial flow has a disadvantage that the feed fluid is not homogeneous, in that the concentration of particles in the feed fluid may increase by many times between the upstream and downstream ends of the feed fluid path. Also, there can be a large pressure drop along the long path, due to friction applied to the moving feed fluid, especially for more viscous liquids. Such large pressure drop can result in the feed fluid pressure being optimum (for maximum permeate flow through the membranes while minimizing fouling) at only a small portion of the total feed fluid path. The flow of the feed fluid in parallel through all of the gaps is seldom used, because the short path length requires repeated return of the fluid for reflow, resulting in large pressure losses during flow near the center of the rotor. A filtration system which allowed the feed fluid to flow along a long path in contact with the membrane surfaces while maintaining the feed fluid largely homogeneous in pressure and particle concentration, would be of value in the filtration of a wide variety of fluids.